Moderate energy (10-100 KV) ion implantation is commonly used in the material processing arts for surface modification of materials and for shallow implants. High energy (0.1 to 2 MeV) ion implantation has recently become a very important process for the placement of atoms further into the bulk material. The energy of an ion accelerated through a potential difference V is given by QV, where Q is the ion charge and V the applied accelerating voltage.
Conventional ion sources are based on either thermionic cathodes or hollow cathode discharges, both of which act as the source of electrons for a plasma discharge. The accelerated electrons in turn create ions by way of electron-gas collisions. These conventional ion sources are characterized by plasma conditions with average electron temperatures, T.sub.e, of 1-10 eV and electron densities of 10.sup.9 to 10.sup.12 electrons/cubic centimeter. Average electron energies at or below 10 eV for the conventional sources produce a low ratio of multiple to single ionized species due to the small overlap of the 10 eV electron energy distribution with the cross sections for multiple ion formation.
The density of multiply ionized atomic or molecular species is so low in conventional ion sourses that the multiple ion current one can extract is typically 10.sup.3 times less than that of single ions. Note that for a multiple ion of charge nQ, acceleration in a potential V results in an energy of nQV. Hence, for equal energies the potential required for an ion of charge nQ is V/n. The use of multiply ionized atomic or molecular species for ion implantation allows for higher energy implantation with a fixed accelerator voltage than when using singly ionized atomic or molecular species. Alternatively, using multiply ionized species nQ, one can use a lower accelerating voltage V/n and still realize the same ion energy as that of a single ion at the full voltage. This condition allows a MeV implanter to operate at 500 KV or a 500 KV implanter to operate at 250 KV if doubly ionized atomic or molecular species are used. Even lower accelerating voltages are possible with higher states of ionization such as A.sup.+3 or A.sup.+4.
For singly ionized atomic or molecular species, the full potential V must be applied to accelerate them to an energy eV. As V exceeds several hundred kilovolts, high voltage design of the acceleration apparatus and associated power supplies becomes costly both in terms of the high voltage requirements on electronic parts and modification necessary to control high voltage breakdown and arcing.